Multi-layer polymer films are widely used in packaging applications, such as bags and pouches for granular, particulate, individual, powder and liquid type products. Some common examples may include cereals, pet foods, snack foods, cookies, chips, seeds, fertilizers, etc. Many of these products are packaged mechanically at high speed for sale or distribution from bulk supplies. Multi-layer films must have the ability to form reliable seals at relatively high speed and appropriate temperature and, in some instances the film must do so in the presence of contamination in the seal region, due to the contents of the package. In many packaging applications involving dry powder or granular products, such as cake mixes, dry soup mixes, powdered drink mixes, hot chocolate mixes, and spices, a preferred packaging method has been a composite, laminated structure comprising a printable, outer paper web that is laminated to a foil layer and includes a second lamination of an inner seal layer on the side of the foil opposite the paper layer. Such incumbent structures are sometimes referred to as a traditional “paper/adhesive/foil/adhesive” structure or sometimes as a “paper/poly/foil/poly” structure. Typically, the “poly” in such structures is a polyethylene polymer.
In such composite packaging structures, the paper layer functions to provide a printable interface and a non-extensible web that can withstand the high stress forces applied as the structure is run through pouch and packaging machines at high speed and in the presence of high temperatures. Additionally, the paper layer functions to withstand the very high sealing temperatures required for sealing the relatively thick composite structures at high speed.
The foil is provided as a barrier layer. Foil has commonly been recognized as a barrier material, having transmission rates with respect to oxygen, water, flavor, aroma, and light of near zero, when in flat, undamaged form. However, in practice the foil layer is subject to flex cracking, pin-holing, crazing, and scratching in the packaging structure, and additionally to further deterioration and damage in the seal area due to the sealing function. The foil is also not very elastic and is subject to damage during film stretching or deformation. These defects may result in severely impaired performance as compared to flat foil substrate performance, leading to decreased shelf life and product spoilage. The failure of the foil layer can occur during the packaging process and during the distribution cycle for storage and retailing.
The inside or product-side adhesive or polymer layer typically functions as the sealable layer and is commonly a sealable polymer containing polyethylene, such as an olefin copolymer or terpolymer containing polyethylene and/or butylene. In some common prior art embodiments the inside sealable layer comprises a layer of extruded, high density polyethylene polymer that provides an acceptable seal strength along with an acceptable degree of leak-proofing in the seal area.
The polymer layer positioned between the paper substrate and the foil layer typically functions as an adhesive to bond the paper and foil. This layer is commonly a coextruded polymer layer, such as polyethylene, that is used as a coextrusion lamination adhesive.
To approach the performance of the paper/poly/foil/poly product without relying upon the fragile foil layer or without the need to perform two extrusions of polymer to laminate and provide a seal layer, some packaging applications may instead utilize just a multilayer polymer film. Flexible packaging polymer films may be treated on an outer surface to permit printability, treated and metallized on an opposing inner surface of the film to provide the barrier performance, and thereafter coated or laminated to an inner polymer seal layer or seal-coating. However, these metallized, coated films are not without their own limitations, such as sensitivity to high temperature at high speed, lack of bond strength of the metallized layer, seal strength concerns, hermeticity concerns, package stiffness concerns, and requiring the separate step of applying a sealant layer to the metal layer. Even with enhanced surface treatment and preparation, the metal layer may be subject to delamination and may require special quality control and handling to ensure consistent application. Further, the metallized layer may still suffer the other deficiencies of foil and may be especially sensitive to scratching and related defects as compared to foil.
Other sealable polymer films useful for packaging are also known in the art. For further example, U.S. Pat. No. 6,624,247 B1 to Kume et al. (Sumitomo Chemical Company, Ltd.) discloses a polypropylene-based heat-sealable film having reduced minimum seal temperature. U.S. Pat. No. 6,641,913 B1 to Hanyu et al. (Fina Technology, Inc.) discloses a multi-layer polyolefin film of the type suitable for packaging application in which heat seals are formed. The multi-layer film comprises a substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interface surface of the substrate layer and is formed of a syndiotactic propylene polymer effective to produce a heat seal with itself at a sealing temperature of less than 110° C. (230° F.). U.S. Pat. No. 6,534,137 B1 to Vadhar (Cryovac, Inc.) discloses a two-component laminated multi-layer film suitable for use in packaging articles, such as pet food, comprising a first component and a non-heat-shrinkable second component. U.S. Pat. No. 5,888,648 to Donovan et al. (Mobil Oil Corporation) discloses a multi-layer film which has an improved composite structure for packages manufactured in a high speed packaging apparatus. U.S. Pat. No. 6,326,068 to Kong et al. (Mobil Oil Corporation) discloses a multi-layer film that has an improved composite structure for providing hermetic seals to packages manufactured in a high speed packaging apparatus. The structure of the multi-layer film includes layers A/B/C/D. Skin layer A is formed from polypropylene copolymer with melt flow rate greater than one or linear high density polyethylene with melt index greater than one. Core layer B is formed from polypropylene. Intermediate layer C has the primary function of compliance during sealing, and sealing layer D has the primary function of providing adhesivity to the completed seal.
Related U.S. application Ser. No. 10/079,662 to Bader, filed on Feb. 20, 2002, discloses a core layer B that comprises a softening additive blended in a core layer to improve the hermeticity of a sealed package. The softening additive enhances compliance of the core layer with the sealable layer while the seal area is heated under pressure within the crimp jaws during sealing operations. The invention of the '662 application functions during sealing operations to effect a more hermetic seal. It is possible to improve hermeticity as per the '662 application without necessarily, substantially improving minimum seal strength.
Though each of the above films represented a variety of improvements related to packaging films, none of the above films combine desired improvements in processability, seal strength, impact strength, hermeticity, durability, and sufficiently reduced seal temperatures for some of today's challenging packaging operations. Opportunities exist for composite, laminated polymer films to replace other packaging composite substrates, such as those including paper and foil, in many demanding packaging operations, such as with ice cream bars, chocolate bars, powders, and other dry-particulate foods.
In many flexible packaging applications, it remains desirable to eliminate the foil layer in the incumbent packaging structures, to provide acceptable barrier properties and hermeticity, to reduce costs and time required to prepare the double-laminated incumbent structures, and to provide a packaging structure that may be microwavable. It is also desirable in some applications to have a polypropylene-based composite packaging structure that does not rely on polyethylene or other adhesive-type polymers for sealability, hermeticity, and seal strength. The inventive composite film meets these and other needs.